Honeycomb of thin material is generally manufactured, in a continuous process, often referred to as an expansion process, as described, for example, in U.S. Pat. Nos. 2,610,934, 2,608,502, 2,734,843, and 2,983,640. Generally in these honeycomb expansion processes, flat sheets are cut from rolls of materials, adhesives are applied in selective lines, strips, or stripes to materials needing a bond adhesive, or masking is undertaken by using non bonding strips, if the materials will bond directly together at the pre-selected non masked lines, strips or stripes. Thereafter the flat sheets closely stacked are so positioned until the selective bonding along the lines, strips, or stripes is completed. Then the block, so formed, of bonded flat sheets, is expanded to form the honeycomb.
Such honeycomb products are used as the core in structural sandwich panels to provide a light weight rigid structure. In many applications where honeycomb is thus used, a combination of structural strength and highly effective heat insulation is often required. In the past this has been partially accomplished by stuffing each cell with fiberous insulation, or by filling the cells with foam. This adding of the insulation has been done subsequently to the manufacture of the basic honeycomb, thereby adding production flow time, and also adding operational disadvantages. Moreover, such insulations may absorb moisture or become contaminated with an undesirable gas or liquid over an extended period of time. This contamination could result from leaks through openings made during manufacture of honeycomb or through damaged areas during use, or through porous areas of a sandwich face sheet, otherwise forming the sealing of a final honeycomb structure.
When hazardous liquids and gases are involved, the detection of small leaks with sensitive indicators is essential. A small leak can be located by pressurizing the honeycomb with a gas that is readily detectable. The honeycomb cells can be interconnected by the use of perforated material used in the manufacture of the basic honeycomb. This permits location of the leak in the sandwich walls. The interconnection of the honeycomb cells also permits cleaning by flushing with an inert gas. Removal of all the contaminates is essential for the detection of future small leaks. The interconnection also permits substitution of air with a low conductivity gas, or evacuation, for improved insulation. This use of interconnection and flushing is taught in U.S. Pat. No. 3,895,152.
In other applications of honeycomb, sealed cells are desired, to isolate and control contamination, as taught in U.S. Pat. No. 3,669,816.
In the case of U.S. Pat. No. 3,895,152, foam insulation, is located in a portion of the length of the honeycomb cell. It is desirable to avoid such insulation, as it will absorb small amounts of contaminates.
Also the effective use of more of the entire length of the honeycomb cell for insulation purposes is desirable. A large part of heat transfer is due to radiation. While foam serves to block this radiation, foam also causes heat transfer by solid conduction, besides having the contamination disadvantage.
Multiple radiation shields made of low emissivity metal foil or metal coated plastic have been used in so called super insulations in turn incorporated into space vehicles. In this super insulation a very large number of radiation shields are separated and supported by low density fiberous mats or webbing. However these separation mats cause solid heat conduction. Also such super insulations have no structural strength, as is needed in many applications to support tanks and to resist structural loads. This necessitates solid structural members bridging such super insulations, thus causing a large heat leak.
Honeycomb has a very high compressive strength to weight ratio in the direction of the hexagonal cell axis. However the open honeycomb cells do not block heat radiation. Heat radiation shields might be inserted in each honeycomb cell and bonded in place after the basic manufacture of the honeycomb. However this would be impractical. Another approach might be to place a radiation shield between slices of honeycomb. However the honeycomb cells on either side of a shield might not register. Therefore a compressive load on the honeycomb would cause the cell walls to cut into each other, unless the heat radiation shield was of sufficient thickness to resist cutting and to transfer the loads from one honeycomb cell to the other.
Therefore the initial incorporation of one heat radiation shield or multiple heat radiation shields within the entire length of each single hexagonal cell and all its adjacent cells, and all honeycomb cells of the resulting structure, as an efficient inserted step in the basic expansion method of the manufacture of honeycomb, would be desirable, and this method and resulting product meet this need.